1. Field of the Invention
This invention relates to a digital signal recording/reproducing apparatus such as a video tape recorder (hereinafter, abbreviated as "VTR"), a video disk player and an audio tape recorder in which video and audio signals are recorded and reproduced in the digital form, and particularly to an apparatus which performs motion-compensation prediction on a video signal for compression-encoding.
2. Description of the Related Art
In a digital VTR for home use, data compression is indispensable in view of cost and hardware size. Hereinafter, therefore, data compression will be described taking mainly a digital VTR for home use as an example.
FIG. 1 is a schematic block diagram showing the structure of a digital VTR for home use. The reference numeral 900 designates an input terminal through which an analog video signal such as a television signal is input. The reference numeral 901 designates an A/D converter which converts the analog video signal into a digital video signal, 902 designates a data compressor which compresses the digital video signal to reduce the information amount of the signal, 903 designates an error-correction encoder which adds error-correcting codes to the coded signal so that errors are corrected in the reproduction, 904 designates a recording modulator which, in order to perform the recording, modulates the signal to codes suitable for the recording, 905 designates a recording amplifier which amplifies the record signal, and 906 designates a magnetic tape on which the record signal is recorded to be stored. The reference numeral 907 designates a head amplifier which amplifies a signal reproduced from magnetic tape 906, 908 designates a reproduction demodulator which demodulates the reproduced signal, 909 designates an error-correction decoder which performs error-correction on the reproduced and demodulated signal using the error-correcting codes, 910 designates a data expander which reconstructs the compressed data to its original form, 911 a D/A converter which converts the digital video signal into an analog video signal, and 912 designates an output terminal.
Next, the data compressor (high-efficiency encoder) 902 will be described. FIG. 2 is a block diagram of the high-efficiency encoder which employs one-way motion-compensation inter-frame prediction. The reference numeral 1 designates an input terminal for a digital video signal, 2 designates a blocking circuit which segments the input digital video signal, 3 designates a subtracter which outputs as a difference block a difference signal between an input block and a prediction block, 4 designates a difference power calculator which calculates the power of the difference block, 5 designates an original power calculator which calculates the AC power of the input block, 6 designates a determiner which compares the difference power with the original AC power to determine whether the current mode is a prediction mode or an intra mode, 7 designates a first switch which selectively outputs an encoded block in accordance with the determined mode, 8 designates a DCT circuit which performs on the encoded block the discrete cosine transform (hereinafter, abbreviated as "DCT") that is an orthogonal transform, 9 designates a quantizing circuit which quantizes a DCT coefficient, 10 designates a first encoder which performs the coding suitable for a transmission path, and 11 designates the transmission path.
Reference numeral 12 designates an inverse quantizing circuit which performs inverse-quantization on the quantized DCT coefficient, 13 designates an inverse DCT circuit which performs the inverse DCT on the inverse-quantized DCT coefficient, 14 designates an adder which adds a prediction block to the decoded block that is an output signal of the inverse DCT circuit 13 to generate an output block, 15 designates a video memory which stores output blocks in order to perform motion-compensation prediction, 16 designates an MC circuit which performs motion estimation from a motion-compensation search block segmented from a past image stored in the video memory 15 and the current input block, and performs motion-compensation prediction, 17 designates a MIX circuit which combines a motion vector with a mode signal determined by the determiner 6, 18 designates a second encoder which codes the output of the MIX circuit 17, and 19 designates a second switch which switches the prediction blocks in accordance with the mode determined by the determiner 6. The difference power calculator 4, original power calculator 5, determiner 6, inverse quantizing circuit 12, inverse DCT circuit 13, adder 14, video memory 15, MC circuit 16 and second switch 19 constitute a local decoding loop 20.
Then, the operation will be described. Irrespective of an intra-field in which motion-compensation prediction is not performed or a prediction-field (inter-field) in which motion-compensation prediction is performed, input digital video signals are divided by the blocking circuit 2 into input blocks in the unit of m [pixels].times.n [lines] (where m and n are positive integers), and segmented. In order to obtain a difference block, the subtracter 3 calculates the difference in the unit of pixel between an input block and a prediction block. Then, the input block and the difference block are input into first switch 7. The difference power calculator 4 calculates the difference power of the difference block. On the other hand, the original power calculator 5 calculates the original AC power of the input block. The two calculated powers are compared with each other by the determiner 6 to control the first switch 7 so that the block having the smaller power is selected as the encoding subject. More specifically, when the difference power is smaller than the original AC power, the determiner 6 outputs a prediction mode signal, and in contrast with this, when the original AC power is smaller than the difference power, the determiner 6 outputs an intra mode signal.
The first switch 7 outputs the input block or the difference block as an encoded block in accordance with the mode signal determined by the determiner 6. When the processed image is in the intra-field, however, the first switch 7 operates so that all of the encoded blocks are output as input blocks. FIG. 3 illustrates this switching operation. The ordinary mode is a mode where, in a step of motion-compensation prediction which is completed in four fields as shown in FIG. 4, first field F1 of the four fields is always an intra-field and the succeeding second, third and fourth fields F2, F3 and F4 are prediction-fields.
The encoded block selected by the first switch 7 is converted into DCT coefficients by the DCT circuit 8, and then subjected to the weighting and threshold processes in the quantizing circuit 9 to be quantized to predetermined bit numbers respectively corresponding to the coefficients. The quantized DCT coefficients are converted by the first encoder 10 into codes suitable for the transmission path 11 and then output to the transmission path 11.
The quantized DCT coefficients also enter into the local decoding loop 20, and the image reproduction for next motion-compensation prediction is performed. The quantized DCT coefficients which have entered into the local decoding loop 20 are subjected to the inverse weighting and inverse quantizing processes in the inverse quantizing circuit 12. Then, the DCT coefficients are converted into a decoded block by inverse DCT circuit 13. The adder 14 adds the decoded block to a prediction block in the unit of pixel to reconstruct the image. This prediction block is the same as that used in the subtracter 3. The output of the adder 14 is written as an output block in a predetermined address of the video memory 15. The memory capacity of the video memory 15 depends on the type of the employed predictive method. Assuming that the video memory 15 consists of a plurality of field memories, the reconstructed output block is written in a predetermined address. A block which is segmented from an image reconstructed from past output blocks and is in the motion estimation search range is output from the video memory 15 to the MC circuit 16. The size of the block in the motion estimation search range is i [pixels].times.j [lines] (where i.gtoreq.m, j.gtoreq.n, and i and j are positive integers). Data in the search range from the video memory 15 and an input block from the blocking circuit 2 are input to the MC circuit 16 as data, thereby extracting motion vectors. As a method of extracting motion vectors, there are various methods such as the total search block matching method, and the tree search block matching method. These methods are well known, and therefore their description is omitted.
The motion vectors extracted by the MC circuit 16 are input to the MIX circuit 17, and combined therein with the mode signal determined by the determiner 6. The combined signals are converted by the second encoder 18 into codes suitable for the transmission path 11, and then output together with the corresponding encoded block to the transmission path 11. The MC circuit 16 outputs as a prediction block signals which are segmented from the search range in the size (m [pixels].times.n [lines]), which is equal to that of the input block. The prediction block to be output from the MC circuit 16 is produced from past video information. The prediction block is supplied to second switch 19, and output from the respective output terminal of the switch in accordance with the field of the currently processed image and the mode signal of the decoded block. Namely, the prediction block is output from one of the output terminals of the second switch 19 to the subtracter 3 in accordance with the processed field, and from the other output terminal in accordance with the mode signal of the current decoded block and the processed field.
As a predictive method used in such a circuit block, for example, the method shown in FIG. 4 may be employed. In this method, an intra-field is inserted after every three fields, and the three intermediate fields are set as prediction-fields. In FIG. 4, first field F1 is an intra-field, and the second, third and fourth fields F2, F3 and F4 are prediction-fields. In the prediction by this method, second field F2 is predicted from first field F1 which is an intra-field, third field F3 is predicted in a similar manner from first field F1, and fourth field F4 is predicted from reconstructed second field F2.
Initially, first field F1 is blocked in the field and subjected to the DCT. Then, first field F1 is subjected to the weighting and threshold processes and quantized, and thereafter encoded. In the local decoding loop 20, the quantized signals of first field F1 are decoded or reconstructed. The reconstructed image is used in motion-compensation prediction for second and third fields F2 and F3. Then, motion-compensation prediction is preformed on second field F2 using first field F1. After the obtained difference block is subjected to the DCT, encoding is performed in a similar manner as in first field F1. In this case, when the AC power of the input block is smaller than the power of the difference block, the input block in place of the difference block is subjected to the DCT, and thereafter encoding is performed in a similar manner as in first field F1. Second field F2 is decoded and reconstructed in the local decoding loop 20 in accordance with the mode signal of each block, and then used in motion-compensation prediction for fourth field F4. In a similar manner as in second field F2, using first field F1, motion compensation prediction and encoding are performed on third field F3. Motion-compensation prediction is performed on fourth field F4 using second field F2 reconstructed in the video memory 15, and then, fourth field F4 is encoded in a similar manner as in third field F3. Also in third and fourth fields F3 and F4, when the AC power of the input block is smaller than the power of the difference block, the input block in place of the difference block is subjected to the DCT, and thereafter encoding is performed in a similar manner as in first field F1.
For example, the digital VTR for home use shown in FIG. 1 is expected to achieve the high image quality and high tone quality. In order to realize this, it is essential to improve data compression, i.e., performance of high-efficiency encoder. Therefore, there arise following problems in the above-described conventional predictive method.
In such a predictive method, since motion-compensation prediction is performed using the video data of the one preceding field or frame, there arises a first problem that the capacity of the field memory or frame memory is increased and the hardware is enlarged in size.
In the conventional predictive method, when a scene change once occurs in the unit of frame, it is difficult during encoding of the image after the scene change to perform the compression according to motion-compensation prediction from the reference picture which was obtained before the scene change, thereby causing a second problem that the total amount of codes is increased. If inter-frame motion-compensation prediction is performed on the whole sequentially in the temporal direction, it may be possible to suppress the increase in the data amount to a minimum level even when a scene change occurs. In the case of encoding interlace images without scene change and with less motion, however, there is a tendency that the data amount is increased as a whole. In a predictive method in which third and fourth fields F3 and F4 are adaptively switched from first, second and third fields F1, F2 and F3 as shown in FIG. 5, there is a drawback that the capacity of the field memory of frame memory is increased and the hardware is enlarged in size. FIG. 6 shows the data amount and S/N ratio of a luminance signal, for example, when an image A with scene changes is processed by the predictive method of FIG. 4 or the predictive method of FIG. 5. In the image A, a scene change occurs in the unit frame. FIG. 6 shows the data amount and S/N ratio of a luminance signal in when an image B without scene changes is processed by the predictive method of FIG. 4 or the predictive method of FIG. 5. In this case, for the image A with scene changes, the predictive method of FIG. 5 is advantageous, and, for the image B without scene changes, the predictive method of FIG. 4 is advantageous.
In the case that the encoding is done by performing prediction as in the prior art, there is a third problem that, when a scene change occurs in a step of a motion-compensation prediction process, the quality of the image immediately after the scene change is deteriorated. This problem is caused as, owing to the scene change, motion-compensation prediction which largely utilizes time correlation is unsatisfactorily performed, thereby increasing the information amount being generated. The information amount generated in this way can compare with the information amount of a usual intra-field in the level. For the generated information amount, the field having this information amount is used as the prediction-field, and therefore, the information amount is compressed to the level of the information amount of the prediction-field, resulting in the image quality of the field after a scene change being substantially deteriorated. FIG. 7 shows a change of the information amount of images for five seconds when encoding is performed by a conventional predictive method. In this case, the average for five seconds is less than 20 [Mbps], but a scene change exists at a position A, thereby increasing the information amount. The change of the S/N ratio in this case is shown in FIG. 8. Although there is no great deterioration in the portion of the scene change, the decrease of the information amount makes the S/N ratio deteriorated. When that field is used in the next motion-compensation prediction, it is necessary to perform motion-compensation prediction on the image with the deteriorated image quality and the reduced time correlation, the result being that the information amount being generated is again increased. This vicious cycle continues until the next refresh field is processed. If deterioration of the image quality occurs in this way, even though it is immediately after the scene change, that means a digital video recording/reproducing apparatus which is required to have a high image quality fails to make the best use of the performance. In a VTR for home use which is one of the digital video recording/reproducing apparatuses, for example, functions such as a trick play and edition are indispensable, and, when such a function is performed, remarkable deterioration occurs in the image quality.
As conventional VTRs for home use of helical scanning type, there are VHS type, .beta. type and 8-mm type VTRs. Hereinafter, a VTR of 8-mm type will be described as an example of a prior art. FIG. 9 is a diagram showing the tape format according to the 8-mm VTR standard (the 8 mm width is indicated as A if FIG. 9) and FIG. 10 is a diagram showing the format of one track. FIG. 11 is a diagram showing the relationship between a rotary head drum and a magnetic tape wound around it, and FIG. 12 is a graph showing the frequency allocation of each signal according to the 8-mm VTR standard. In an 8-mm VTR for the NTSC system or PAL system, a video signal is recorded by a color undermethod which is a basic recording method for VTRs for home use. The luminance signal is frequency-modulated with a carrier of 4.2 to 5.4 MHz, chroma signal subcarrier is converted into a low frequency signal of 743 kHz, and the both signals are subjected to the frequency multiplex recording. The recording format on a tape is as shown in FIG 9. All signals required for a VTR at least including a video signal (luminance signal, color signal), audio signals and tracking signals are subjected tot he frequency multiplex recording by the rotary video head. The frequency bands are shown in FIG. 12.
In FIG. 9, magnetic tracks 401 and 402 of a video signal track portion 410 are tracks for a video signal, and each corresponds to one field. Letters A, B, C, D, E and F designate tape width (8 mm); distance to the center of a video track (4.461 mm); guard band width (0.1 mm); width of video, FM audio, and tracking signals (5.351 mm); width of PCM audio (about 1.25 mm); and audio track width (0.6 mm), respectively. Magnetic tracks 403 and 404 indicated with oblique lines in an audio signal track portion 411 are magnetic tracks for audio signals. A cue track 405 and audio track 406 for a fixed head are respectively set on the both edges of the tape. Since the control track on the tape edge is not used in an 8-mm VTR, this track can be used as the cue track for performing specific point searching, addressing the contents of recording or the like. The width of one track (track pitch) is 20.5 .mu.m, and is slightly greater than that in the economy recording mode of .beta. type and VHS type (19.5 .mu.m in .beta.-7, 19.2 .mu.m in the 6-hour mode of VHS). No guard band for preventing a crosstalk from occurring is set between tracks. Instead, azimuth recording using two heads is employed in order to suppress a crosstalk.
Next, a specific example of the operation of a conventional apparatus will be described with reference to FIGS. 13 to 16. FIG. 13 is a block diagram of a conventional VTR. A video signal given to a video signal input terminal 201 is supplied to a video signal processing circuit 203 and also to a synchronizing signal separating circuit 204. The output signal of the video signal processing circuit 203 is fed through gate circuits 205 and 206 to adders 213 and 214. In contrast, a vertical synchronizing signal which is an output of the synchronizing signal separating circuit 204 is supplied to delay circuits 207 and 208. The Q output of the delay circuit 207 which combines with the synchronizing signal separating circuit 204 is constitute head switch pulse generation means is supplied as a gate pulse to the first gate circuit 205 and also to a fourth gate circuit 212 which will be described later. The Q output is supplied as a gate pulse to the second gate circuit 206 and also to a third gate circuit 211 which will be described later. The output signal of the delay circuit 208 is supplied to a time-base compressing circuit 209 and also to an erasing current generator 240.
An audio signal given to an audio signal input terminal 202 is supplied through the time-base compressing circuit 209, a modulating circuit 210 and a switch 241 for switching between the recording and the erasing, to the third and fourth gate circuits 211 and 212. The output of the erasing current generator 240 is supplied through the switch 241 to the third and fourth gate circuits 211 and 212. The output signal of the third and fourth gate circuits 211 and 212 are supplied to the adders 213 and 214, respectively. The output signal of the adder 213 is given to a rotary transformer 217 through a changeover switch 215 for switching between the recording and the erasing. The output signal of the rotary transformer 217 is given to a rotary magnetic head 221 through a rotation shaft 219 and a rotary head bar 220, so that a recording current or an erasing current flows into a magnetic tape 223.
The output signal of the adder 214 is given to a rotary transformer 218 through a switch 216 which is used for switching between recording and the erasing and is interlocked with the switch 215. The output signal of the rotary transformer 218 is given to another rotary magnetic head 222 through the rotation shaft 219 and the rotary head bar 220, so that a recording current or an erasing current flows into the magnetic tape 223. The magnetic tape 223 is guided by guide posts 224 and 225 placed on the either sides of a table guide drum 226 which has rotary magnetic heads 221 and 222 built in, and is run at a constant speed in the direction of arrow 227, by a well known magnetic tape running device (not shown) which consists of capstans and pinch rollers. The table guide drum 226 may have a well known structure, and therefore its specific description is omitted.
In the reproduction process, a signal reproduced by the rotary magnetic head 221 is supplied to a separating circuit 228 through the rotary head bar 220, the rotation shaft 219, the rotary transformer 217 and the switch 216. On the other hand, a signal reproduced by the rotary magnetic head 222 is supplied to a separating circuit 229 through the rotary head bar 220, the rotation shaft 219, the rotary transformer 218 and the switch 216. One of the outputs of the separating circuit 228 and one of the outputs of the separating circuit 229 are supplied to an adder 230. The other output of the separating circuit 228 and the other output of the separating circuit 229 are supplied to an adder 231. The output signal of the adder 230 is supplied to a video signal output terminal 233 through a video signal processing circuit 232. In contrast, the output signal of the adder 231 is supplied to an audio signal output terminal 237 through time-base correcting circuit 234, a demodulating circuit 235 and a time-base expanding circuit 236.
Then, the operation will be described. A video signal given to the video signal input terminal 201 is converted into an FM signal by the video signal processing circuit 203. When the video signal includes a chrominance signal, the chrominance signal is converted into a low frequency signal of less than about 1.2 MHz. There will be no problem that, for example, the phase of the chrominance signal is shifted by 90 deg. or inverted every 1H (horizontal scanning interval) as means for eliminating an adjacent color signal. This is a technique of eliminating a crosstalk between tracks with using the line correlation of chrominance signal. Such processed video signal is supplied to first and second gate circuits 205 and 206.
On the other hand, since the video signal is given also to the synchronizing signal separating circuit 204, a vertical synchronizing signal is obtained at the end of the output of the circuit. The vertical synchronizing signal is supplied to the delay circuits 207 and 208. The delay circuit 207 has functions of dividing an input signal into a half frequency and delaying a signal. From the ends of Q and Q outputs of the delay circuit 207, pulse signals Q and Q for switching the heads and shown in FIGS. 14(b) and 14(c) are supplied to the first and second gate circuits 205 and 206, respectively. In order to clarify the relationship in phase between these pulse signals Q and Q and the input video signal, the waveform of the input video signal is shown in FIG. 14(a). From the ends of outputs of first and second gate circuits 205 and 206, the processed video signals are output as shown in FIGS. 15(a) and 15(b) during the periods in which the pulse signals Q and Q are at H level. These signals are added to a modulated compressed audio signal and erasing signal which will be described later, by adders 213 and 214, and then supplied to switches 215 and 216. The compressed audio signal is subjected to modulation suitable for the tape and head system (preferably, the pulse code modulation (PCM), or FM, PM, AM or the like, or in certain cases, the non-modulation AC bias recording), by the modulating circuit 210. Particularly, PCM is advantageous because a high S/N ratio can be expected and well known error correction means can be used for the drop-out, etc. The modulated compressed audio signal is given through the switch 241 to the third and fourth gate circuits 211 and 212 to which the pulse signals Q and Q are respectively supplied. These gate circuits 211 and 212 output the compressed audio signal to the adders 213 and 214 during the periods in which the pulse signals Q and Q are at H level.
The erasing current generator 240 generates an erasing current of a certain frequency (for example, 100 kHz). The timing of starting the oscillation of the erasing current is controlled by a trigger signal T which is obtained by delaying the vertical synchronizing signal in the delay circuit 208. The erasing current is output through the switch 241 to the third and fourth gate circuits 211 and 212 to which the pulse signals Q and Q are respectively supplied, and supplied to the adders 213 and 214. In the same manner as the recording of compressed audio signals, during the periods in which Q pulse signals Q and Q are at H level. FIGS. 16(a) and 16(b) shows the waveforms of the output currents of the adders 213 and 214, i.e., time-multiplexed signals of a processed video signal A and a processed audio signal B or the erasing signal. These signals are supplied via the above-mentioned paths to the rotary magnetic heads 221 and 222, whereby the magnetic pattern of a tape shown in FIG. 9 is obtained.
During the reproduction process, the moving contacts of the switches 215 and 216 are positioned at fixed contacts P. This allows the two-channel reproduction signal reproduced by the rotary magnetic heads 221 and 222 to be respectively transmitted through the rotary head bar 220, the rotation shaft 219, the rotary transformers 217 or 218 and the switches 215 or 216, and to be respectively separated into a video signal and an audio signal on the time-base in separating circuits 228 and 229. The separated video signals are converted by the adder 230 into a one-channel video signal which is continuous in terms of time, and then supplied to the video signal processing circuit 232. The video signal processing circuit 232 reconstructs the original video signal from the input signal, and outputs the reconstructed signal to the video signal output terminal 233. On the other hand, the separated audio signals are converted into a one-channel of signal by the adder 231, and then supplied to the time-base correcting circuit 234. The time-base correcting circuit 234 consists of a semiconductor memory device such as a CCD (charge-coupled device) and a BBD (bucket brigade device), and eliminates time-base variations (so-called jitter and skew distortion) of the tape and head system. The output signal of the time-base correcting circuit 234 is demodulated to the original compressed audio signal by the demodulating circuit 235. The demodulated signal is then converted into the original audio signal by the time-base expanding circuit 236 consisting of a semiconductor memory device such as a CCD and a BBD, and output to the audio signal output terminal 237.
As described above, in an 8-mm VTR, video signals and audio signals for one field are recorded on and reproduced from one track on a tape.
FIG. 17 is a block diagram showing the configuration of a conventional video information recording/reproducing apparatus. In FIG. 17, a digital VTR of the D1 or D2 method which is used for business or broadcasting use is shown. The reference numeral 101 designates an A/D converter which converts an analog video signal into a digital video signal, 102 designates an error-correction encoder which adds error-correcting codes, 103 designates a modulator which modulates the digital signal to a signals suitable for the recording on a magnetic tape, 104 designates a rotary head drum, 105 designates a magnetic tape, 106 designates magnetic head for recording and reproduction, 107 designates a demodulator which demodulates the reproduced signal, 108 designates an error-correction decoder which detects and corrects a transmission error, and 109 designates a D/A converter which converts the digital video signal into an analog video signals.
FIG. 18 shows the tape formats of the two methods. In the both methods, a video signal and a 4-channel audio signal are recorded in different positions in the same track. In the D1 method, an audio signal is recorded in the center of a track, and, in the D2 method, at the ends of a track. When a video signal and an audio signal are recorded in the same track, components such as a magnetic head and an amplifying circuit which are necessary for recording and reproducing can be used in common for a video signal and an audio signal, and furthermore, a parity code required for the error correction as described later and a circuit for generating the parity code can be used in common.
FIG. 19 shows the overall specifications of the D1 and D2 methods, FIG. 20 shows the specifications of the tape formats, and FIG. 21 shows the specifications of the tape running systems. The area recording density with guard bands being taken into account is 21.5 .mu.m.sup.2 /bit in the D1 method, and 16.6 .mu.m.sup.2 /bit in the D2 method. In the D1 method, guard bands are set between recording tracks, but, in the D2 method, guard bands do not exist. As a result, the track density of the D2 method is higher than that of the D1 method by about 15%, which contributes to the long-time recording by the D2 method. On the other hand, when guard bands do not exist, it is more likely to reproduce a signal of an adjacent track in addition to a signal of the track originally intended to be reproduced. In order to cope with this crosstalk between tracks in the reproduction process, the D2 method employs the azimuth recording system. Generally, a recording magnetic head and a reproduction magnetic head are so positioned that their head gaps form the equal angle with a magnetic track. If the two head gaps are arranged so as to form an angle with each other, the level of a reproduced signal shows an attenuation characteristic. The azimuth angle .theta. in the D2 method is about .+-.15 deg. as shown in FIG. 20. As a result, even if a signal from an adjacent track is mixed in signals, to be reproduced the unnecessary component is attenuated. Accordingly, even if guard bands do not exist, the effect of the crosstalk is reduced. Since the loss due to the azimuth angle cannot be expected for DC components, however, signals to be recorded are required to have no DC component. Therefore, the D2 method employs a modulation system which does not include DC components.
In a digital recording, it is not necessary to record a video signal during the entire period. In a blanking interval, a video signal has a constant waveform irrespective of the contents of an image. Since this waveform can be synthesized after the reproduction, in both the D1 and D2 methods, the recording is performed only during the effective video period. Also a color burst signal appearing in a blanking interval of an NTSC signal can be synthesized after the reproduction. This is because the sampling phase in the D2 method is set to the I and Q axes and the phase of the color burst (lagging behind Q axis by (180+33) deg.) can be determined using a reproduced sampling clock.
FIG. 22 shows the ranges in which pixels can be actually recorded in the D1 and D2 methods. These effective pixels are divided into several segments. In the D1 method, pixels of 50 scanning lines constitute one segment, and, in the D2 method, pixels of 85 scanning lines constitute one segment. In other words, pixels of one field constitute five segments in the D1 method, and three segments in the D2 method.
When a video signal in a segment is to be recorded, it is divided into four channels in the D1 method, and into two channels in the D2 method. As a result, the number of pixels per one channel of one segment is {(720+360.times.2)/4}.times.50=360.times.50=18,000 in the D1 method, and (768/2).times.85=384.times.85=32,640 in the D2 method. Channels are distributed so that they are uniformly dispersed on a screen. Accordingly, even when the characteristics of a specific channel are deteriorated, code errors caused by this deterioration are not concentrated on one portion of the screen so as to be inconspicuous. Therefore, the effect of correction on errors which have not been corrected is also enormous.
In both the D1 and D2 methods, two kinds of error-correction codes which are respectively called an outer code and an inner code are used together. In an actual process of generating outer and inner codes, an operation of rearranging the sequence of the codes is performed. This operation is called shuffling. The shuffling disperses the effect of code errors, improves the correction capability, and reduces the display deterioration caused by errors which have not been corrected. The shuffling process consists of the shuffling for one scanning line which is performed before the generation of an outer code, and the shuffling which is performed in one sector after the addition of an outer code and before the generation of an inner code. As described above, in a VTR of the D1 or D2 method, video signals and audio signals for one field are recorded in a plurality of tracks on a tape.
In order to record all information of standard television signals of currently used NTSC and PAL systems, in a VTR for home use, the carrier frequency of an FM luminance signal is raised and the bandwidth and deviation are increased so as to improve the resolution and C/N ratio. However, a VTR for home use still fails to match with a VTR for business use in S/N ratio, waveform reproducibility, etc. The down sizing of a VTR is highly expected to be achieved, and, there are demands for further improvement in performance as well as realization of VTR which is light and compact. Hence, it is difficult to attain the desired performance by only improving the present techniques. On the other hand, in the field of VTRs for business use and broadcasting use, rapid advance in digitalization of an apparatus has been made to achieve multifunction and high performance in the apparatuses, and most of VTRs for broadcasting use are replaced with digital VTRs. However, a digital VTR consumes a large amount of tape, which obstacles to achieve prolongation of the recording time and the down sizing.
Recently, in view of the redundancy of information contained in an image, studies on compressing recorded information have been actively conducted, and the application of the results of these studies to a VTR is being examined. It is expected to realize a VTR which is compact and light, has high image quality and can operate long-time by achieving high image quality, and reduction of tape consumption due to high density recording and information compression which are inherent in the digital recording.
FIG. 23 shows a communication apparatus of a high-efficiency encoded video information compression method (according to CCITT H. 261, etc.) which is used in the field of communication including a video telephone and a video conference. The reference numeral 101 designates an A/D converter which converts an analog video signal into a digital video signal, 110 designates a high-efficiency code encoder which compression-encodes a video signal, 112 designates a buffer memory which is used for delivering generated compressed codes at a constant speed, 102 designates an error-correction encoder which adds error-correcting codes, 103 designates a modulator which modulates the digital signal to a transmission signal suitable for the communication, 114 designates a transmission path, 107 designates a demodulator which demodulates a received signal to a digital signal, 108 designates an error-correction decoder which detects and corrects a transmission error, 113 designates a buffer memory which is used for supplying compressed codes that have been received at a constant speed, in accordance with the request from the next stage, 111 designates a high-efficiency code decoder which expands the compressed video signal to the original signal, and 109 designates a D/A converter which converts the digital video signal into an analog video signal.
The redundancy of an input video signal always varies, and therefore, the amount of codes which are compression-coded using this redundancy also varies. However, the amount of information which can be transmitted through the transmission path 114 is limited. In order to exhibit the best of the performance, the variation of the code amount is buffered using the buffer memory 113, and the information amount is controlled to be within a predetermined range so that overflow or underflow of a memory does not occur. FIG. 24 shows the buffer operation performed at the receiving side. Data which have been received at a constant rate are stored in the buffer memory, and, when the data amount reaches the level B0, decoding of the codes starts. At the time when data of d1 have been consumed for the display of the first picture and the decoding for the second picture starts, the amount of the accumulated data is B1. In the same manner, data accumulation and data consumption are alternately repeated. The amount of consumed data varies depending on the displayed picture, but the average amount of consumed data is equal to the receiving rate. The operation of the receiving side has been described. The operation of the transmitting side is performed in the entirely opposite way to that of the receiving side.
Since the communication apparatus is controlled as described above, the relationship between fields of an input video signal and transmitted codes is not clearly defined. Unlike an application in the field of communication, a VTR is required to perform functions peculiar to a VTR and including a special reproduction different from normal reproduction such as a still reproduction, slow reproduction and high-speed reproduction, an assemble edition, and an insert edition. Therefore, it is desirable to clearly define the relationship between fields and tracks. In order to produce a practical VTR, it is essential to select a recording format which can solve these problems.
As a method of compressing a moving picture such as a television signal, there is a method using an intra-field (or intra-frame) in which the encoding is completed within an individual field (or frame) independently of another field (or frame), and a prediction-field (or prediction frame) in which the predictive encoding is performed using information of another field (or frame). Generally, the information amount of the intra-field (or intra-frame) in which the prediction between fields (or frames) is not used is two or more times the code amount of the prediction-field (or prediction frame) in which the encoding is performed using the prediction between planes. When record areas of the same size (number of tracks) are allocated to the intra-field (or intra-frame) and the prediction-field (or prediction frame), therefore, there arises a fourth problem that in the intra-field (or intra-frame), the record area is not sufficient and in the prediction-field (or prediction frame), the record area has a useless portion.
As described previously, encoding of video signals in a high-efficiency encoder by means of motion compensation prediction may be carried out either of two encoding modes: an intra mode wherein only the information in the screen to be encoded is used (motion compensation prediction is not conducted), or a prediction mode wherein information in other screens is also used (motion compensation prediction is conducted). It is known that the efficiency of video compression is low in the intra mode which does not utilize the information in other screens and is about 50% that of the prediction mode wherein information in other screens is also utilized. Therefore increasing the ratio of prediction mode operation to decrease the ratio of intra mode operation as far as possible may be conceived as a means for improving the efficiency of compressing the entire information. However, because the reproduction in prediction mode is carried out on the assumption that the video information to be referred to is completely reproduced, it cannot be avoided that special reproduction such as high-speed reproduction or editing by means of VTR depends predominantly on the reproduction of intra mode. As a consequence, there is the fifth problem that the ratio of intra mode operation cannot be decreased at will, which makes an obstacle in increasing the compression efficiency.